Redefining Biology via Enzyme Engineering

A TV series from the 1970s, ‘‘The Six Million Dollar Man,’’ imagines a former astronaut with bionic implants and superhuman strength acting as a secret agent for the government. Such enhancement of human abilities with artificial components (all at a reasonable price tag) might resonate with some of today’s scientists who are working on a more modest goal of augmenting the properties of cells and biological systems. Their aim is to design new synthetic routes for compounds and even create completely new molecules that may not exist in nature by exploiting the versatility of the life’s systems. Take the current work on silicon. Silicon is a ubiquitous element that sits next to carbon in the periodic table and shares some of its properties. The similarities in the ways silicon and carbon bond with other atoms have led to the idea that silicon might give rise to an alternative, siliconbased life with unique properties. Yet, silicon does not readily partake in biochemical couplings unless nudged to bind carbon via chemical synthesis. Organosilicon compounds that are thus created are highly valued for their ability to bind organic and inorganic substrates and are used as adhesives, sealants, and caulks. Their industrial synthesis is laborious, and thus the interest in bringing silicon into the fold of nature’s biochemistry persists. Now, advances in enzyme engineering have made it possible to incorporate silicon into organic compounds through bacterial biosynthetic pathways. Frances Arnold and her team (Kan et al., 2016) have discovered that a cytochrome c protein from the thermophilic bacteriumRhodothermusmarinus can, under the right conditions, catalyze the formation of a carbon-silicon bond to create short organosilicon compounds. The examination of the enzyme’s structure yields clues to the motifs driving this process, allowing the rapid directed evolution of an enzyme with high activity for making such products. Strikingly, this altered cytochrome c churns out silicon-containing compounds inside of bacteria, suggesting that relatively few